This invention relates to optical systems for spectrophotometers and, more particularly, to such systems wherein a light beam is passed through a cuvette and then directed onto the photosensitive surface of a photoelectric detector.
It will be appreciated that for accurate measurement, the light beam should always illuminate the same area of the photosensitive surface, because the various areas of this surface, in general, have different sensitivities. Thus, signal variation may result if a light beam is displaced relative to the photosensitive surface. The sample, itself, affects the light beam, as the cuvette filled with the sample is arranged in the path of the light rays. The windows in the cuvette, through which the beam passes, may not be mutually plane parallel, which results in what is known in the art as wedge error. Such a wedge error causes angular deflection of the light beam, but it can be avoided by manufacturing the cuvette in an appropriately precise manner.
In addition, errors result when the individual rays of the light beam are offset, or out of parallel, due to the cuvette with its windows not being mounted normal to the axis of the beam. Parallel offset may also be caused by the beam converging at the location of the cuvette, as the individual rays impinge on the window at an angle which differs from 90.degree., whereby each ray experiences a corresponding parallel offset. This phenomenon also affects the extent of the area of the photosensitive surface impinged upon by the beam. It will be appreciated that this type of error is particularly aggrevating because the parallel offset of the individual converging beams cannot be avoided even by exact alignment of the cuvette and, in addition, it depends on the variable indices of refraction of the samples. It is noted that the indices of refraction of a sample to be tested and of a reference sample, measured earlier or subsequently, may deviate considerably from each other even within the range of the absorption bands of the sample.
It is well known to arrange the cuvette in a collimated portion of the light beam. See, for example, Kortum, "Kolorimetrie, Photometrie Und Spektrometrie" 4th edition, 1962, page 136, FIG. 69b. In this prior art arrangement, the parallel light beam was subsequently focused on a monochromator entrance slit to form an image of the light source. It required a rather large cross-sectional area of the collimated beam portion at a correspondingly large cross-sectional area of the cuvette to transmit a given light flux, which resulted in an undesirably large sample volume. If the monochromator entrance slit of the prior art arrangement was replaced by a photosensitive surface of a photodetector, an image of the light source would be formed on the photosensitive surface but this surface would not be uniformly illuminated over a large area. Such an arrangement would be extremely sensitive to any kind of angular deflection of the collimated light beam, which would result in displacement of the small, bright light with respect to the photosensitive surface of the detector, and hence move it to an area having a different sensitivity.